1. Field of the Invention
The present invention relates to methods of exploring for oil or gas deposits. Particularly, the present invention relates to methods for predicting the likelihood of finding oil or gas deposits at unexplored depths by utilizing temperature and geothermal gradient information available from shallower depths. More particularly, the present invention relates to methods of predicting the presence of oil or gas deposits by comparing the temperatures and/or the geothermal gradients at one location with average values of temperatures and geothermal gradients within the geographical area around the location.
2. Description of the Background
Oil and gas deposits are commonly found in sedimentary earth formations from about the surface of the earth to depths of about 25,000 feet or more. Within these sedimentary formations, it is known that the temperature of the earth steadily increases with depth from a point near the surface. The surface of the earth, and very near-surface, are affected by seasonal fluctuations in atmospheric and benthic temperatures. Consequently, at the near-surface limit to which seasonal temperature changes reach, the temperature of the earth is in many cases constant at about the value of the yearly average surface temperature. From this near surface limit point, the temperature of the earth begins its steady increase proportional to depth. As a result of this temperature increase with depth, curves of temperature vs. depth may be constructed, which indicate a surface temperature and a temperature at any depth within the interval over which the curve is constructed. The "surface temperature" on such a temperature vs. depth curve is generally about equivalent to the yearly average surface temperature. Also, a change in temperature with change in depth can be measured and is called the "geothermal gradient" which is expressed in units of (.degree.F./100 ft.) or (.degree.C./100 m). The geothermal gradient is equivalent to the mathematical slope of the temperature vs. depth curve.
Data showing the temperature of the earth at various depths has been obtained for a very large number of locations, both throughout the United States and the world. For example, when electric-logs or nuclear-logs are obtained on wells, the bottom-hole depth and temperature are measured and recorded upon the well-log record. Much of this temperature-depth data from well-logs is available to the public from the various state agencies which are responsible for oil and gas drilling. In the state of Texas, such information is available from the Railroad Commission of the State of Texas.
Geological surveys often include surface and near-surface earth temperature data. Meteorological surveys include average annual atmospheric temperature data; marine surveys include benthic temperature data. Results of such surveys are publically available from such sources as the U.S. Geological Survey and the National Atmospheric and Ocean Services.
In many geographical areas, local geologists and geophysicists have adopted values for surface temperature and for geothermal gradient which are representative of the average of these values within the geographical area. These adopted values have been arrived at by empirical means and are not notably accurate when applied to a specific location within the geographical area. However, such representative values of surface temperature and geothermal gradient are well known to those exploring for oil and gas in the geographical areas, and such representative values are useful in circumstances where no better temperature-depth data is available.
In most instances, the average geothermal gradient for a geographical area can be represented by a single value for depth ranges of many thousands of feet. The average temperature vs. depth curve for a geographical area can be represented as a straight line passing through the scattered or actual temperature vs. depth measurements. However, in some geographical areas, the average geothermal gradient cannot be accurately represented by one constant value over the depth interval of interest. In such cases, the average geothermal gradient is assigned one value over one depth interval, and a second value over a second depth interval. One particular circumstance where the average geothermal gradient is given different values over different depth intervals is where a geopressurized zone is found at some depth below the surface. Earth formations within geopressurized zones are thought to be subject to pressures and earth stresses which increase heat conduction from below, consequently increasing temperatures within these zones at rates much greater than rates of temperature increase experienced at shallower depths. Thus, in geographical areas where geopressurized zones are thought to exist, the geothermal gradient is given a first, lower value over the interval from the surface to the top of the geopressurized zone, and a second, higher value over the depth interval below the top of the geopressurized zone.
Those skilled in exploring for oil and gas recognize that the values which represent the average values of surface temperature, geothermal gradients and temperature vs. depth information for a particular geographical area will not be accurate when applied to a specific location. It is well known that measured values of temperature and geothermal gradient at specific locations can vary substantially from the average values developed for the surrounding geographical area.
Wells at some locations may have anomalously high temperatures at certain depths, compared to the average for wells in the surrounding geographical area. Wells having anomalously high temperatures at substantial depths often also have anomalously high pressures at such depths, corresponding with the concept of geopressurized zones. In addition to geopressure effects, anomalously high temperatures in wells often result from other causes, e.g. the presence of thermally conductive intrusives such as salt domes. Proposals have been made that anomalously high temperatures may be encountered around the periphery of oil deposits, forming a "halo" of high temperatures outlining the oil deposit. Studies of temperatures around known oil fields have been made for the purpose of demonstrating the presence of these halos of high temperature. The published results of these studies show mixed results, with halos of high temperatures delineating some oilfields but not others.
Wells may also have anomalously low temperatures at some depths, compared to average temperatures for the surrounding geographical areas.
In situations where direct measurement of temperature and depth in a well are made, e.g. in well logging operations, two sources of error are predominant: the temperature measured is inaccurate because the thermometer is not given time to equilibrate before being moved; the well may have horizontal deviations such that the measured depth is greater than the actual depth. Both these sources of error tend to produce measured temperature values which are lower than the true temperature values.
Those skilled in the art of exploring for oil and gas know that heat is transmitted at different rates through different earth formations. It is also known that the geothermal gradient in a given well varies according to the type of earth formation being traversed.
We have observed that temperatures and geothermal gradients for the same depth intervals vary significantly from well to well, even within small geographical areas. Further, such actual values of temperatures and geothermal gradients vary significantly from the values which represent the averages for the geographical area. From these observed variations in temperature and geothermal gradient from well to well, and even within the same well, we have discovered improved methods for predicting the likelihood that oil or gas deposits are present below selected locations.